2.6 Pretreatment: Screening, Presedimentation, Aeration & Preoxidation
Key Takeaways
- Presedimentation removes heavy silt and sand ahead of coagulation, cutting coagulant demand and sludge volume during high-turbidity river events.
- Aeration strips carbon dioxide, hydrogen sulfide, methane, and volatile taste-and-odor compounds while adding oxygen to oxidize dissolved iron and manganese.
- Potassium permanganate oxidizes iron, manganese, and taste-and-odor compounds without forming trihalomethanes, but overdosing produces pink water and manganese dioxide post-precipitation in the distribution system.
- Prechlorination of high-TOC raw water maximizes trihalomethane and haloacetic acid formation because free chlorine contacts organic precursors before they are removed by coagulation.
- Copper sulfate applied for algae control is typically dosed at 0.5 to 1.0 mg/L and works poorly in high-alkalinity water because copper precipitates as basic copper carbonate before reaching the algal cells.
Pretreatment: Screening, Presedimentation, Aeration & Preoxidation
The ABC Water Treatment outline lists "source water treatment (e.g., algae control, aeration, mixing)" and "pretreatment" as separate monitor-evaluate-adjust tasks ahead of coagulation. Pretreatment is where an operator buys down chemical cost, filter loading, and disinfection byproduct formation before the main treatment train ever sees the water.
1. Intake Screening
Raw water intakes carry debris, fish, leaves, ice, and zebra or quagga mussels. The screening train usually runs coarse to fine:
- Trash racks / coarse bar screens - 1 to 3 inch clear openings, manually or mechanically raked, sized so approach velocity stays low enough (typically 0.5 ft/s or less at the screen face) that fish are not impinged and headloss stays small.
- Traveling water screens - continuous belts of wire mesh panels (3/8 inch mesh is common) with a spray wash header that flushes debris into a trough. Differential level across the screen is the control signal: rising differential means the mesh is blinding and rotation speed or wash pressure must increase.
- Fine screens or microstrainers - 20 to 60 micron fabric used where algae carryover is a chronic filter-loading problem.
Zebra mussel control at intakes relies on continuous low-dose chlorination, periodic thermal or chemical shock, and antifouling coatings. Veligers settle inside intake piping, then adult shells reduce cross-section and slough into the plant.
2. Presedimentation and Desilting
Presedimentation is plain gravity settling of raw water without coagulant, ahead of the rapid mix. It is used on rivers with flashy turbidity - the James, the Roanoke, the Potomac after a storm - where raw turbidity can jump from 15 NTU to 800 NTU in hours.
What presedimentation buys you:
| Benefit | Mechanism |
|---|---|
| Lower coagulant dose | Removes the heavy, easily settled fraction (sand, coarse silt) that would otherwise consume coagulant and sweep floc |
| Smaller sludge volume in the main basins | Grit is captured in a basin designed to be dredged, not in a clarifier designed for light floc |
| Flow and quality equalization | Detention (often 3 to 8 hours) buffers rapid raw-quality swings so coagulant control loops are not chasing a moving target |
| Protection of downstream equipment | Abrasive sand is removed before it reaches chemical mixers and pumps |
Presedimentation basins are frequently earthen or concrete with sloped floors and either a mechanical scraper or a periodic dewater-and-excavate cycle. Some plants add a small polymer or coagulant dose ahead of presedimentation during extreme events - once you do, the process is technically a first-stage coagulation, and the sludge changes character from grit to chemical sludge.
3. Aeration
Aeration transfers gases in both directions: stripping unwanted gases out and dissolving oxygen in. The direction that dominates depends on which gas is out of equilibrium with the atmosphere.
What aeration accomplishes
- Strips carbon dioxide - raising pH, which reduces the lime or caustic needed for corrosion control or softening.
- Strips hydrogen sulfide - eliminating rotten-egg odor from groundwater. Effective only at low pH, where H2S is present as dissolved gas rather than as bisulfide ion.
- Strips methane and volatile organics - radon, TCE, and similar VOCs from groundwater.
- Adds dissolved oxygen - oxidizing ferrous iron to filterable ferric hydroxide, and to a much lesser degree manganese.
- Removes some taste-and-odor compounds, though geosmin and MIB are poorly stripped because they are not very volatile.
Common aerator types
| Type | Description | Best for |
|---|---|---|
| Cascade / multiple tray | Water falls over weirs or through stacked coke or slat trays | CO2 and H2S removal, iron oxidation; simple, low maintenance |
| Spray / nozzle | Water sprayed into open air | CO2, H2S; high land and head requirement |
| Diffused air | Compressed air bubbled through a basin | VOC stripping, in-basin mixing |
| Packed tower (air stripper) | Countercurrent air through packing media | High-efficiency VOC and radon removal |
Operational caution: aeration raises dissolved oxygen and pH, both of which can shift the corrosion balance and can precipitate iron and manganese in the wrong place. Aerating manganese-bearing groundwater without a downstream filter simply moves the black stain from the customer's laundry to the clearwell wall.
4. Preoxidation
Preoxidation applies an oxidant upstream of coagulation to accomplish iron and manganese oxidation, taste-and-odor destruction, color removal, algae control, or improved coagulation.
| Oxidant | Strengths | Watch-outs |
|---|---|---|
| Free chlorine | Cheapest, provides residual, oxidizes iron and sulfide fast | Maximizes TTHM and HAA5 because it contacts NOM before coagulation removes it; oxidizes manganese slowly at pH below 8 |
| Potassium permanganate (KMnO4) | Excellent for iron, manganese, geosmin and MIB; forms no halogenated byproducts | Pink water if overdosed; the reaction product MnO2 must be filtered out or it post-precipitates as black water in the mains |
| Ozone | Most powerful oxidant used in water treatment; superb for taste, odor, and color; no chlorinated byproducts | Forms bromate in bromide-bearing source water; produces biodegradable organic carbon that can regrow bacteria unless followed by biofiltration; no residual |
| Chlorine dioxide (ClO2) | Oxidizes manganese quickly across a wide pH range; does not react with ammonia; low THM formation | Forms chlorite and chlorate; combined residual limit for ClO2 plus chlorite is regulated; generation on site requires careful chemistry |
The permanganate dose rule of thumb
Stoichiometrically, roughly 1 mg/L KMnO4 oxidizes about 1 mg/L of iron and roughly 2 mg/L KMnO4 oxidizes about 1 mg/L of manganese. Because raw water also exerts an oxidant demand from organics, operators set dose by jar test to a faint pink endpoint that disappears before the filters, not by stoichiometry alone. A persistent pink at the filter influent means the dose is too high.
The prechlorination trade-off
Prechlorination is the single most common cause of Stage 2 D/DBPR compliance trouble at conventional plants. Moving the chlorine application point from the raw water to the settled water or filter effluent - so free chlorine contacts water only after coagulation and sedimentation have removed 40 to 70 percent of the TOC - is usually the cheapest available DBP reduction step. If prechlorination is retained for zebra mussel or algae control at the intake, the dose should be the minimum that achieves that purpose.
5. Reservoir and Algae Control
Algal blooms drive taste-and-odor complaints, shorten filter runs, raise coagulant demand, elevate pH through photosynthesis, and - with cyanobacteria - can release cyanotoxins such as microcystin.
Control strategies
- Copper sulfate (CuSO4 * 5H2O) - the traditional algaecide, typically applied at 0.5 to 1.0 mg/L of copper sulfate over the affected area, usually by boat-towed drag bags or spray. Effectiveness collapses in high-alkalinity water: above roughly 40 to 50 mg/L alkalinity as CaCO3, copper precipitates as basic copper carbonate and settles before contacting cells. Chelated copper formulations are used to keep copper in solution. Copper is toxic to fish, accumulates in sediment, and lyses cells - releasing intracellular cyanotoxins - so it should not be used during an active toxic bloom without treatment capability downstream.
- Reservoir destratification / hypolimnetic aeration - mixing or oxygenating the bottom layer prevents the anoxic release of soluble iron, manganese, phosphorus, and sulfide, and reduces the nutrient supply that drives blooms.
- Multi-level intakes - shifting the withdrawal depth to avoid the algae-rich epilimnion in summer or the anoxic hypolimnion during stagnation. This is the least expensive control an operator can exercise day to day.
- Watershed nutrient control - the only durable solution: limiting phosphorus loading from agriculture, stormwater, and wastewater discharges.
- Powdered activated carbon (PAC) - applied at the rapid mix or presedimentation basin, typically 5 to 30 mg/L, to adsorb geosmin, MIB, and dissolved toxins. PAC must be applied before chlorine or it is wasted consuming the oxidant, and it must have enough contact time before being removed with the floc.
A conventional surface water plant is exceeding its Stage 2 D/DBPR locational running annual average for TTHM. Raw water TOC averages 4.5 mg/L and the plant prechlorinates at the raw water intake. What process change is most likely to reduce TTHM formation?
An operator applies potassium permanganate ahead of the rapid mix for manganese removal and observes a persistent faint pink tint in the filter influent and occasional black water complaints in the distribution system. What is the most likely cause?
A utility plans to apply copper sulfate to control an algal bloom in a raw water reservoir with a total alkalinity of 140 mg/L as CaCO3. What outcome should the operator anticipate?